If you mean by opening a business is being a shareholder of a company then yes you can. You don't even need to be in Thailand to be a shareholder of a company. You may attend business meetings time to time or visit suppliers or trade shows. However, you cannot be part of day to day business operations and definitely cannot work. if your business just sources the product in Thailand and sell them overseas and make the money overseas then it can fall under overseas income. However, if you're selling products or services within Thailand and making money within Thailand then it's different. Basically you can only attend meetings or visit suppliers or trade shows but cannot be more involved directly daily. Best talk to an visa tax agent/lawyer. Cheers